There are services that a plain old telephone system (POTS) can sustain while executing many sorts of system maintenance. One of these services is the ability to maintain loop line voltage at a customer premises to keep customer premise equipment (CPE) devices operating and unaffected by the system maintenance.
A subscriber line circuit in a plain old telephone system (POTS) is a circuit connecting a subscriber line at a customer premises to the central office. The subscriber line is traditionally a twisted-pair loop for carrying analog voice. Today, the subscriber line is also used for digital services as well, such as provided via cable, DSL or satellite.
Customer premise equipment (CPE) devices, such as phones, facsimiles, home security systems, answering machines, etc., connected to the subscriber line at the customer premises may depend on the line voltage of the subscriber line for proper operation. For example, an alarm system at a customer premises monitors the line voltage. If the line voltage falls below a threshold for a predetermined period of time, the alarm system assumes the subscriber line has been cut, and sends an alarm message to a remote monitoring station. In addition, the alarm system activates a loud, local alarm siren, which can wake everyone in the middle of the night for a false alarm if the line voltage falls below a threshold in the middle of the night.
The POTS has the ability to maintain line voltage to keep the CPE devices operating and unaffected when system maintenance is performed, such as software downloads and upgrades, system reboots, and system re-configurations. However, the ability to maintain line voltage during system maintenance may not be supported by a multimedia terminal adaptor (MTA) for a voice-over-internet-protocol (VoIP) network. VoIP is a digital telephone service that may be provided through a broadband connection, such as a coaxial cable from a cable service provider, and typically replaces conventional POTS service provided by a telephone company over a twisted pair network. VoIP may be deployed at the customer premises via the MTA. The MTA interfaces with an IP network and is operable to adapt VoIP data for use by CPE devices connected to the subscriber line. The MTA may be embedded in a modem, such as a cable modem or DSL modem, as an embedded MTA or may be provided as a standalone device connected to the modem.
Maintenance is periodically performed on the modem or MTA, or the VoIP service provider's network, which may include software downloads, correcting a lock-up situation or other maintenance. During a maintenance period, a network clock signal generally provided by the service provider's network during normal service is generally not provided to the modem or MTA, hence preventing the modem or MTA from being able to generate a line voltage for the SLIC. Once the maintenance is completed, the modem or MTA must be reset to lock onto the network clock signal, and also possibly to run a new program installed during the maintenance. The line voltage on the subscriber line may drop because the MTA does not include the ability to maintain the line voltage when during the maintenance nor during a reset/resync operation to the network clock. This may result in failure or improper operation of a CPE device. For example, if the line voltage falls below a threshold for a predetermined period of time, the alarm system assumes the subscriber line has been cut, and sends a false alarm to the remote monitoring station. Furthermore, scheduled system maintenance is typically performed during non-peak hours, such as late at night. Thus, a false alarm caused by the scheduled maintenance may appear real, because home robberies mostly occur during the late night hours.
A specialized SLIC device may be able to generate a line voltage without the network clock; however, the SLIC itself must also reset to synchronize onto the new network clock. Resetting of the SLIC also generally results in a loss of line voltage. The current generation of VoIP Subscriber Line Interface Cards (SLICs), is not capable of re-synchronizing to a new input clock. These current generation devices must be fully reset, then taken out of reset, so that the new input clock can be processed. This causes a major disruption to the SLIC's main purpose, which is generating line voltage for telephony devices.